This invention relates to the art of catheters and more particularly to a catheter having means for locating and then ablating cardiac conduction pathways.
The heart, in a human, is a four chamber muscular organ that pumps blood through various conduits to and from all parts of the body. In order that blood be moved in the cardiovascular system in an orderly manner, it is necessary that the heart muscles contract and relax in an orderly sequence so that the valves of the system open and close at proper times during the cycle. The control is initiated by a special structure known as a sino-atrial node (SA node). This is the natural pacemaker of the heart and is specialized tissue located within the muscle walls of the right atrium. Basically, the SA node provides what may be considered dominance over the inherent or natural rhythmic contractions of the atria and ventricles. This dominance is transmitted by ionic impulses through cardiac conduction pathways in the atria and the ventricles thereby causing the heart to contract and relax in an orderly sequence at a rate dictated by the SA node. The sequence insures that each ventricular contraction maximizes the volume of blood flowing to the pulmonary and systemic circulation. The SA node has an inherent rate or rhythm which can be modified by the action of the sympathetic and parasympathetic nervous system.
The impulses are transmitted from the SA node through the atria to the atrio-ventricular node (A-V node) and, thence, to the ventricles by way of cardiac conduction pathways (Bundle of His and Purkinje fibers). The A-V node transmits the impulses by way of a common pathway, also known as the Bundle of His and, thence, by way of two lower branches to a network of fibers which cover the inside of each ventricle. This conductive network extends to the outer covering of the heart and is called the Purkinje system. The ionic or current flow in the cardiac conduction pathways may be interrupted or altered by disease which can cause the formation of scar tissue. When injury occurs in the cardiac conductive pathways or to the microcardium, the electrical impulses as dictated by the SA node are not transmitted normally, then rythmic disturbances can take place in the heart which are called cardiac arhythmias (dysrhymthmias). A principle means used by physicians for analysis of cardiac dysrhythmias is the electro-cardiogram.
The term bradycardia is used to describe an abnormal slowing of the cardiac contractions. Tachycardia is a term used to describe excessive rapidity of heart action. Tachycardia dysrhythmia or tachydysrhythmia may impose substantial risk to a patient because diseased hearts cannot usually tolerate such rapid rates for extensive periods. Thus, when there is a marked underlying heart disease, such rapid rates may cause hypotension and heart failure. Tachycardia in those patients with underlying cardiac disease can degenerate into a more serious ventricular dysrhythmia such as fibrillation.
It is therefore desirable in situations of abnormal tachycardia, resistant to medical management, to terminate this rapid rate by ablating at least a portion of the cardiac conduction pathway either to decrease the heart rate or to disrupt the orderly sequence partially or totally.
If the origin of the tachycardia is above the ventricles it is termed Supra Ventricular Tachycardia (SVT). SVT is only life threatening if the atrial rate is exceedingly high and the atrial activity is conducted to the ventricles in a 1:1 ratio. If during an episode of SVT the ventricular rate was 1/2 of the atrial rate, there would be no threat to survival. Thus, it may be desirable to create (via ablation) second degree heart block (wherein the ventricular rate is related to the atrial, but only 1 in every n beats are conducted). For example, if the atrial rate was 250 b.p.m. (beats per minute), the ventricular rate would be a safe 125 b.p.m. if there were chronic 2:1 second degree block. A drawback of this is that at rest the ventricular rate may be dangerously slow in the absence of an artificial pacemaker. Thus, a more desirable state may be to ablate sufficient tissue to cause only first degree heart block at rest. This implies slowed conduction through the AV node. At higher rates, as in SVT's, the degree of block progresses to second because the atrial depolarization occurs during the relative refractory period of the ventricles. So at rest there is 1:1 conduction and at higher rates there are occasional "dropped beats" or 2:1 or 3:1 block or more.
Additionally, where a patient suffers from brady/tachy syndrome, the abnormal slowness of the pulse rate may be such that the stimulation of the heart muscles can be better controlled as with an implantable electronic pacemaker and, in such cases, it may be desirable to ablate the conduction pathways in conjunction with implanting such a pacemaker.
Ablation of a portion of ventricular tissue may be sufficient to prevent the occurrence of ventricular tachycardia. Elimination of abberent ventricular conduction pathways should prevent VT's.
Additionally, in conducting research for the development of pacemakers and other implantable pulse generators, it is necessary to experiment with animals. In such case, in conjunction with implanting an experimental pulse generator, it may be necessary to locate and ablate a cardiac conduction pathway.
The techniques employed in the prior art, such as described in the patent to Bures U.S. Pat. No. 3,865,118, have included the difficult operation of surgically sectioning the His Bundle to produce A-V block. Bures proposed that the SA node be destroyed so that the heart rate may drop abruptly without the necessity of destroying the His Bundle, during the implantation of a pacemaker. In either event, however, Bures contemplates opening the chest to have access to the heart muscle.
It is therefore desirable to provide apparatus and method for ablating the cardiac conduction pathways without the necessity of surgically opening the patient's chest to provide access to the heart muscle. Instead, it is desirable to provide means which may be nonsurgically inserted into the heart muscle to locate and ablate such pathways.
Catheters are known in the art and generally take the form off an elongated tubular element having at least one lumen extending throughout its length. These catheters have a distal end which is inserted into a cavity, artery or the like within a patient's body and a proximal end located externally of the patient's body. Such catheters are known wherein fiber optic means extend from the proximal end to the distal end so that a surgeon may deliver light to a cavity or the like and view the illuminated cavity by means located at the proximal end of the catheter. Additionally, some catheters also carry additional fiber optic means for transmitting a laser beam to the illuminated cavity for applying laser energy to the body tissue thereat. Such a catheter is disclosed, for example in the M. Bass U.S. Pat. No. 3,858,577. However, there is no suggestion in Bass that the catheter carries some means, such as electrodes or the like, responsive to electrical impulses transmitted by the cardiac conduction pathways for purposes of locating such pathways and also some means for ablating the thus located cardiac conduction pathways.
Catheters are known, however, which carry electrodes and such is illustrated in the A. R. Bures U.S. Pat. No. 3,865,118, supra. However, Bures employs the electrodes for purposes of stimulating heart muscles, including both atrial and ventricular stimulation. No suggestion is provided in Bures of employing electrodes for purposes of locating cardiac conduction pathways as well as some means for ablating the thus located pathways.